SINGLE NERVE CELLS

SINGLE NERVE CELLS

It is laughable to think that anyone completely understands how the brain functions. Every time neuroscientists make a discovery that explains some property of the nervous system, that discovery opens new doors and raises new questions. For example, no one knows exactly how the CNS stores memories, but we do know a lot about how to alter the

storage process.

Often the brain is compared to a computer. That analogy is overworked, but it's not such a had one. Most people know how to use a computer, and they know that smashing the disks is a bad idea, but they do not know precisely how the circuits inside the computer do the job. However, not knowing just how the circuits work does not prevent the user from know­ing where to insert the disk, how to turn on the monitor, and how to run a program. Likewise, there is a lot to know about the nervous system, and a little knowledge can help you keep yours healthy.

The first step is to appreciate what a miraculous structure the brain is. The real miracle is that such a complex structure can function so well even under some of the terribly difficult conditions that we impose on it. It has an ingenious balance of excitatory and inhibitory influences coursing through it. It's like a sports car moving along a winding coun­try road with just the right amount of pressure on the accelerator (exci­tation) and the brakes (inhibition). In the brain, the brakes are the release of inhibitory chemicals. They suppress the firing of nerve cells by opening channels in the cells' membranes, letting ions flow in a direc­tion that causes the cells' electrical potential to move away from the point at which it would fire a signal (an action potential). Without action potentials, there is no action, so we say that that cell or network of cells is inhibited. An inhibited network cannot carry out its function, so that function is lost. The lost function might be thinking, feeling anxiety, staying awake, having reflexes to pain, adjusting the circulatory system, or breathing. An overly excited network is like a pot of boiling water, or like that sports car out of control at high speed. There is a chaos of dis­charges that randomly fire in many parts of the brain, leading to all sorts of feelings and movements. It is a miracle that in most of us, for

most of the time, the brain maintains the delicate balance that permits a normal life.

The first step to understanding that delicate balance, and how drugs disrupt it, is to understand the building blocks of the CNS—the nerve cells, or neurons. There are many other CNS cells that support the neurons, but the neurons are where the information is stored, where feelings are sensed, and where actions are initiated.

Neurons look a little like trees. Did you ever see a big tree uprooted? There's the trunk and the top with many branches and the leaves that receive the sunlight. Then there is the root system that is equally branched, with a large taproot going off into the earth. Under the microscope, many neurons look the same way. They have a "top" receiving area called the dendrites, where connections from other neurons make contact. Then they have a "trunk" area, where the body of the nerve cell is located, con­taining the genetic information for that cell. Finally out of the cell body emerges the axon of the cell (like the root of a tree), which goes off and

branches to make contact with other nerve cells or muscle cells and trans­mit signals to them.

Like all cells, a nerve cell is held together by its cell membrane, which is a mixture of lipids (fats) and proteins. Many nonneuronal cells (e.g., blood cells, muscle cells) have cell membranes that are more or less the same all over. The cell membranes of neurons, however, are vastly different in dif­ferent parts of the cell. These differences allow a cell to receive different types of signals from many other cells, integrate these signals, and then send out signals of its own. Even a single neuron is a very complicated bit of biochemical machinery, but this complexity is what allows the enormous information storage and processing capacity of the human brain to exist in such a compact form.